The hypothesis of this proposal is that MgS04 decrease myometrial contractility by interfering with the stimulatory effect of Ca2+ at intracellular locations. To test this hypothesis, the effects of MgS04 on specific steps in the contractile response of human myometrium will be evaluate; these studies will include: 1) simultaneously evaluating contractile force and cytosolic Ca2+ to determine the relationship between the ability of MgS04 to decrease contractility and to inhibit increases in cytosolic Ca2+; 2) determining if MgS04 inhibits activation of phospholipase C; 3) determining if MgS04 inhibits agonist-induced activation of protein kinase C; and 4) determining if MgS04 inhibits activation of myosin light chain kinase. For these studies strips of myometrium and cultured uterine myocytes will be obtained from pregnant women undergoing cesarean section deliveries, which are uncomplicated by infection or pharmacologic treatment for premature labor.